Designing Games with Game Maker - YoYo Games

Attractors
Besides emitters a particle system can also contain attractors. An attractor attracts the
particles (or pushes them away). A particle system can have multiple attractors. You are
though recommended to use few of these because they will slow down the processing of the
particles. An attractor has the following properties:
• x,y indicate the position of the attractor.
• force indicates the attracting force of the attractor. How the force acts on the
particles depends on the following parameters.
• dist indicates the maximal distance at which the attractor has effect. Only particles
closer that this distance to the attractor will be attracted.
• kind indicates the kind of attractor. The following values exist
o ps_force_constant indicates that the force is constant independent of the
distance.
o ps_force_linear indicates a linearly growing force. At the maximal distance
the force is 0 while at the position of the attractor it attains the given value.
o ps_force_quadratic indicates that the force grows quadratic.
• additive indicates whether the force is added to the speed and direction in each step
(true) or only applied to the position of the particle (false). When additive the particle
will accelerate towards the attractor while with a non-additive force it will move there
with constant speed.
The following functions exist to define attractors. Note that each of them gets the index of the
particle system to which it belongs as a first argument.
part_attractor_create(ps) Creates a new attractor in the given particle
system. It returns the index of the attractor. This index must be used in all
calls below to set the properties of the attractor.
part_attractor_destroy(ps,ind) Destroys attractor ind in the particle
system. Call this if you don't need it anymore to save space.
part_attractor_destroy_all(ps) Destroys all attractors in the particle
system that have been created.
part_attractor_exists(ps,ind) Returns whether the indicated attractor
exists in the particle system.
part_attractor_clear(ps,ind) Clears the attractor ind to its default